There has been known an axial motor constructed such that magnetic poles of stators and permanent magnets disposed on a rotor are arranged to face each other in directions parallel to a rotary shaft. Such an axial motor has advantages that miniaturization is possible and also an output can be increased. Thus, the axial motor is used in various applications.
The axial motor is constructed, for example, as disclosed in patent literatures 1 and 2. More specifically, the axial motor includes a rotor having a plurality of fan-shaped permanent magnets arranged in an annular manner on the surface of an iron core made of soft iron or the like, and stators with coils which form magnetic poles facing these permanent magnets in directions parallel to a rotary shaft. Directions of the magnetic poles of the respective permanent magnets (directions connecting N-poles and S-poles) are parallel to the rotary shaft, and adjacent permanent magnets are so arranged that the directions of the magnetic poles are opposite to each other. By generating a rotational magnetic field by the coils of the stators in the axial motor constructed as above, magnetic attractive and repulsive forces are produced on the respective permanent magnets, with the result that a torque is generated to rotate the rotor.
Such an axial motor has a problem that torque pulsation (cogging torque) is likely to occur. A method for preventing this problem is proposed, for example, in patent literature 3. Patent literature 3 discloses an axial motor set such that, out of contour lines of coils and those of permanent magnets, those in directions crossing a rotor rotation direction are substantially straight and the contour lines of the coil and those of the permanent magnets move toward and away from each other in the rotor rotation direction in a non-parallel manner while the rotor is rotating. In this axial motor, when the rotor rotates, a magnetic flux variation caused when magnetic fields by the permanent magnets pass magnetic fields by the coils of the stators gradually changes as the rotor rotates. As a result, sudden torque pulsation is suppressed to alleviate a cogging torque.
Further, since the axial motor is structured to obtain a motor torque by the action of only magnetic torques by the permanent magnets, the amount of the permanent magnets needs to be increased to increase a torque. Thus, if a desired torque is large, the volume of the axial motor increases, leading to a problem of size enlargement. Further, cost also increases as the axial motor becomes larger. Accordingly, various proposals have been made for a method for preventing such problems, i.e. a method for reducing the amount of permanent magnets necessary to increase a torque in an axial motor. For example, patent literature 4 discloses an axial motor constructed such that magnetic materials are arranged between permanent magnets in a rotor. By providing the magnetic materials between the respective permanent magnets in this way, the amount of the permanent magnets can be reduced by as much as the magnetic materials are provided. Although a magnetic torque decreases by as much as the permanent magnets are reduced, it becomes possible to utilize a reluctance torque by providing the magnetic materials, wherefore a total motor torque can be maintained. In this way, the amount of the permanent magnets can be reduced without reducing the torque as a whole.
Patent literature 4 further proposes an axial motor constructed such that magnetic materials are provided at least on outer surfaces of permanent magnets. By adopting such a construction, the reluctance torque can be further increased. This enables a torque reduction to be further suppressed as a whole while the amount of the permanent magnets is reduced.
An axial motor with a reduced amount of permanent magnets is realized by a technology disclosed in the above patent literature 4. However, higher output and higher efficiency have been required also for axial motors in terms of resource saving and energy saving in the wake of recent increasing consciousness on environmental problems.